The invention lies in the integrated technology field and relates, more specifically, to an ESD protective circuit for an integrated circuit, which is integrated in a semiconductor substrate of the integrated circuit, having a lateral pnpn xe2x80x9clatch-upxe2x80x9d protective structure formed by a first region, which is doped with a first doping type, is arranged in a well doped oppositely, with a second doping type, and is provided with a first connection electrode, the well being inserted into the semiconductor substrate doped with the first doping type, and also by a second region, which is doped with the second doping type, is arranged outside the well in the semiconductor substrate and is provided with a second connection electrode, and having a highly doped zone of the second doping type, which is arranged on the well boundary, at the same time partly overlapping the well, along that section of the periphery of the well which runs between the two regions.
An ESD protective circuit of that type is disclosed in the reference book by Amerasekera and Duvvury, xe2x80x9cESD in Silicon Integrated Circuitsxe2x80x9d, John Wiley and Sons, Chichester (1995).
Integrated circuits, primarily the sensitive circuits using complementary MOS technology (CMOS), are often intended to be protected against damage which can be caused by an electrostatic discharge (ESD). An ESD event can result in a voltage breakdown through a dielectric that isolates two surfaces, that is to say ultimately a short circuit, which may lead to damage for example in oxide layers or on interconnects or connections of the integrated circuit. The electrostatic charging of the circuit that precedes this sudden discharge is usually brought about through contact between the circuit and an electrically charged human body or a charged machine.
In order to afford protection against such ESD events, it is customary to integrate protective circuits on the semiconductor substrate of the circuit to be protected, which, when a hazardous discharge current and/or a discharge voltage occurs, are activated, assume a low-impedance state and draw off the discharge current at least to an extent such that the sensitive regions of the circuit to be protected remain safe. So far, circuits having lateral pnpn xe2x80x9clatch-upxe2x80x9d structures have proved to be the most effective ESD protection.
The xe2x80x9clatch-upxe2x80x9d effect, which has been known per se for a fairly long time, was originally perceived only as an undesirable error source in CMOS circuits. Almost all CMOS circuits have a lateral and a vertical parasitic bipolar transistor which together form a four-layer diode pnpn, comparable to a thyristor. The four-layer structure can be triggered in the event of interference of the applied supply voltage. This transition from the normal state of the pnpn structure to a highly conductive state is referred to as xe2x80x9clatch-upxe2x80x9d. The integrated circuit can be thermally overloaded by the current path formed in an undesired manner between the transistor connections in this context.
A structure which is similar but is produced in a desired manner and in addition to the actual integrated circuit and is based on elements arranged laterally in the substrate is currently used, as already mentioned, as ESD protection and is described in detail in sections 4.2.4 to 4.3.1 of the aforementioned reference book. This known latch-up protective structure (Lateral Silicon Controlled Rectifier, LSCR) or its modified embodiment (MLSCR) that is also described therein will now be explained, in order to provide a better understanding of the invention, in connection with FIG. 3 (cf. FIGS. 4.35 and 4.31 in Amerasekera and Duvvury, supra).
FIG. 3A shows a schematic cross-sectional view of a prior art MLSCR ESD protective structure. There, an n-conducting well 2 is inserted into a weekly p-doped substrate 1. A highly p-doped region 3 and, in order to connect the n-type well 2 to the supply voltage, a highly n-doped region 4 are produced in the n-type well 2. A further highly n-doped region 5, which is connected to ground, is produced outside the n-type well 2. This configuration already produces a lateral pnpn structure:
Region 3, n-type well 2, p-type substrate 1, region 5,
which forms the basic LSCR protective structure. What is crucial for the triggering voltage of the pnpn protective structure is the pn junction between the p-type substrate 1 and the n-type well 2. A triggering voltage of about 50 V or less results for contemporary typical CMOS process parameters.
With the ever greater miniaturization of the structures on the semiconductor chips and the associated danger caused even by smaller voltages, there is a need for ESD protective circuits which react even to low voltages, for example below 50 V. In order to reduce the triggering voltage, the cited literature therefore proposes additionally arranging the highly n-doped zone 6xe2x80x94illustrated in FIG. 3Axe2x80x94above the well boundary between the n-type well 2 and the p-type substrate 1. By virtue of this measure, the triggering voltage of the thus modified LSCR structure (MLSCR) in typical 0.8 xcexcm CMOS processes is virtually halved and the parasitic capacitance is increased. The highly doped n-type zone 6 proposed is embodied as strips on the well boundary.
FIG. 3B shows the layout of the circuit of FIG. 3A in a schematic view from above. The regions 3 and 4 situated within the n-type well 2, and the region 5 situated laterally outside the n-type well 2 are shown. The strip-shaped highly doped n-type zone 6 on that section of the periphery 7 of the n-type well 2 which runs between the regions 3 and 5 is intended to have the effect that the lateral latch-up structure used as protective structure triggers both statically and dynamically in the case of an ESD event at a lower voltage than previously, and thus also sooner than other, unintentional parasitic latch-up structures within the integrated circuit, in order thus to avoid a failure of the circuit due to interconnects melting, increased leakage currents, and the like.
Although the use of the MLSCR achieves the lowering of the triggering voltage that is sought, it is also known from the cited literature and it has been confirmed in practical tests that the lowering of the triggering voltage with the aid of the n-type zone 6 is accompanied by the disadvantage of a higher on resistance of the pnpn protective structure after triggering. The consequence of this, for example for a chip which, with the LSCR, exhibited failures above 2 kV (ESD test according to the human body model), is that failures occur at as little as 1.5 kV when the MLSCR is used. In accordance with the cited literature, the increased on resistance is brought about precisely by the n-type diffusion strip 6 which is used to lower the triggering voltage and lies directly in the low-impedance (after triggering) path of the pnpn structure.
The object of the present invention is to provide an ESD protective circuit which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which results in a low triggering voltage, but without an increased on resistance.
With the above and other objects in view there is provided, in accordance with the invention, an ESD protective circuit for an integrated circuit in a semiconductor substrate having a doping of a first doping type, comprising:
a lateral pnpn xe2x80x9clatch-upxe2x80x9d protective structure formed by
a first region with a doping of a first doping type arranged in a well formed in the semiconductor substrate and having a doping of a second doping type opposite the first doping type, and formed with a first connection electrode;
a second region with a doping of the second doping type arranged outside the well in the semiconductor substrate, and formed with a second connection electrode; and
a highly doped zone formed of a plurality of mutually spaced-apart pads with a doping of the second doping type, the pads being disposed to partly overlap the well and to cover a well boundary along a section of a well periphery running between the first region and the second region.
In other words, the objects of the invention are satisfied in the case of an ESD protective circuit of the type mentioned in the introduction by virtue of the fact that the highly doped zone is designed in the form of pads arranged with intermediate spacing along the section of the periphery of the well.
The effect achieved by the ESD protective circuit according to the invention with its optimizable layout is that the triggering voltage decreases, as in the case of the known MLSCR, and a low on resistance occurs at the same time, said on resistance virtually corresponding to the value in the case of the LSCR. To that end, in contrast to the known MLSCR, a continuous highly doped strip zone is not implanted above the well boundary, but rather merely a plurality of highly doped pads with intermediate spacings. In gap regions between the implanted pads, the current path after triggering is at low impedance, as in the case of the LSCR, while the function of lowering the triggering voltage is fulfilled to a sufficient extent by the highly doped pads arranged between anode and cathode of the pnpn structure, along the pn junction of n-type well/p-type substrate. The two desired effects are thus achieved simultaneously in one and the same section of the periphery of the well. This opens up opportunities for skillful configuration of the layout, in particular with regard to minimization of the area taken up by the ESD protective circuit within the integrated circuit.
In accordance with the present invention, it is unimportant here whether the first doping type is p-type doping and the second doping type is n-type doping, or vice versa.
In accordance with an added feature of the invention, the xe2x80x9clatch-upxe2x80x9d protective structure is constructed mirror-symmetrically from two lateral bipolar transistors formed by
two laterally adjacent regions doped with the first doping type and a central region doped with the second doping type and disposed between the two laterally adjacent regions, the two laterally adjacent regions and the central region being disposed within the well and each being connected to a first pole of a supply voltage via a connection electrode; and
by two regions doped with the second doping type and respectively disposed laterally adjacent the two laterally adjacent regions outside the well in the semiconductor substrate; and
the highly doped pads are disposed with an intermediate spacing along two laterally mirror-symmetrically opposite sections of the periphery of the well.
In other words, this advantageous refinement of the ESD protective circuit with a particularly skillful layout has the xe2x80x9clatch-upxe2x80x9d protective structure constructed mirror-symmetrically from two lateral bipolar transistors formed by two regions, which are arranged laterally next to one another and are doped with the first doping type and between which is arranged a central region doped with the second doping type, all three regions being arranged within the well and each being connected to a first pole of a supply voltage by means of a connection electrode, and by two regions, which are doped with the second doping type and are arranged laterally, that is to say in each case next to the regions, and outside the well in the semiconductor substrate. The highly doped pads are arranged along the two laterally mirror-symmetrically opposite sections of the periphery of the well.
In accordance with an additional feature of the invention, the pads are arranged along the sections of the periphery of the well with regular intermediate spacings short enough for uniform triggering of the xe2x80x9clatch-upxe2x80x9d protective structure over an entire length of the sections.
In accordance with another feature of the invention, the first doping type is p-type doping. In the alternative, the first doping type is n-type doping.
With the above and other objects in view there is also provided, in accordance with the invention, an integrated circuit with the above-outlined ESD protective circuit. The integrated circuit has input/output and ground connections and the ESD protective circuit is connected between the input/output connection and ground connection of the integrated circuit in such a way that, in the event of a sufficiently intense electrostatic discharge at the input/output connection, the ESD protective circuit triggers, assumes a low-impedance state, and thereby draws off or absorbs a discharge current of the electrostatic discharge. Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an ESD latch-up protective circuit for an integrated circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.